In general, the skin is divided into the epidermis, the dermis and the subcutaneous tissue when viewed from the exterior, and functions to protect the whole organs in the body from variations in temperature and humidity, ultraviolet rays, and other external physical and chemical environmental irritations. Particularly, the epidermis has an important role in preventing evaporation of moisture in the human body.
The epidermis is divided into the stratum corneum, the stratum granulosum, the stratum spino and the stratum basale. Additionally, keratinocytes present in the stratum corneum serve as bricks and intercellular lipids present between keratinocytes serve as mortar, thereby forming skin barriers (J. Invest. Dermatol. 80 (Suppl.) 44-49, 1983). Further, a high concentration of natural moisturizing factors (NMF) is present in keratinocytes of healthy humans so that the skin retains moisture. For example, amino acids, which are water soluble materials, are effectively bound to moisture, thereby inhibiting moisture from drying on the skin (J. Invest. Dermatol., 54, 24-31, 1970).
However, skin drying and roughening phenomena, caused by a drop in moisture content of the stratum corneum and including loose, dry and inanimate skin conditions, occur due to various causes. Such causes include artificial temperature control for cooling/warming an indoor space depending on variations in the living environment and pattern, diverse stresses generated from social activities and skin stresses caused by environmental pollution, frequent face-washing depending on makeup habits, and natural skin aging. Therefore, there has been an increasing need for a skin moisturizing agent.
According to the prior art, a humectant capable of absorbing moisture or an occlusive moisturizer capable of preventing moisture evaporation have been used as moisturizing agents so as to increase moisture retainment in the stratum corneum. Such humectants include glycerin, propylene glycol, 1.3-butylene glycol, polyethylene glycol, sorbitol, sodium 2-pyrrolidone-5-carboxylate, or the like. However, such humectants have a disadvantage in that they have a highly sticky and dense feel when applied on the skin. Additionally, as the occlusive moisturizers, lipids such as ceramides or essential fatty acids and lipid complexes have been used (J. Invest. Dermatol. (5), 731-740, 1994). However, such occlusive moisturizers have difficulty in maintaining stability of an emulsified formulation and are not amenable to production of transparent gel-like cosmetic products.
Meanwhile, peroxisome proliferator activated receptors (PPAR) are known nuclear hormone receptors having the three isoforms of α, β/δ and γ distributed over various tissues. Peroxisome proliferator activated receptor isoform α (also referred to as ‘PPAR-α’) has been identified first based on the mechanism of controlling genes encoding fatty acid oxidase by the reaction with a peroxisome proliferator factor such as a fibric acid derivative (Issemann and Green, Nature, 1990, 347: 645-650). Additionally, it is disclosed that fatty acids play an important role in tissues expressed by PPAR-α (Leone et al., Proc. Natl. Acad. Sci. USA, 1999, 96: 7473-7478). Lipid activators of PPAR-α, such as linoleic acid, etc., are known to those skilled in the art. It is demonstrated that such activators enhance formation of an epithelial barrier in vitro (Hanley et. al., J. Clin. Inv., 1977, 100: 705-712). However, in the relevant art, there is no disclosure of a skin cosmetic agent utilizing a mechanism of controlling PPAR activities other than a cosmetic composition (WO01/008653) for preventing and treating skin aging.